It started today and I haven't restarted yet to check if it's only a temporary problem. I'm on Mac (10.13.4) with the newest Firefox Quantum (59.0.2) and am using PopUp Blocker and OneTab as plugins. I was on a website, and needed to open about 20 subpages each in their own tab. It worked, but some of the tabs show a completely black screen, except for a tiny white box in the lower left corner. Apparently the site is still active, sometimes I can still see an ad, and the cursor shifts between a text-cursor and the usual arrow. Scrolling the black site sometimes turns the screen white. When I looked through the 20+ tabs I had just opened, I noticed that it was quite regularly every 4th tab that had turned black. Also a new tab that I opened to look for this support site was black at first, because it happened to be 4 tabs after another black one. I am attaching a screenshot here. The funny thing is: the screen was black when I took it. Now in the *.png file it is white. I haven't had this kind of bug before and am worried if this is some malware, a simple bug in Firefox. I tried opening some other websites now, and on every 4th tab I get a monochrome screen. For Facebook it's actually a very light-grey or -blue (and nothing visible on it). What could it be?